The present invention relates to papermaking, and more particularly to belts used in papermaking. Belts of the present invention can reduce energy consumption and improve the drying rate required for thermal drying of paper fibers formed on a three dimensional belt.
Cellulosic fibrous structures, such as paper towels, facial tissues, napkins and toilet tissues, are a staple of every day life. The large demand for and constant usage of such consumer products has created a demand for improved versions of these products and, likewise, improvement in the methods of their manufacture. Such cellulosic fibrous structures are manufactured by depositing an aqueous slurry from a headbox onto a Fourdrinier wire or a twin wire paper machine. Either such forming wire is an endless belt through which initial dewatering occurs and fiber rearrangement takes place. Frequently, fiber loss occurs due to fibers flowing through the forming wire along with the liquid carrier from the headbox.
After the initial formation of the web, which later becomes the cellulosic fibrous structure, the papermaking machine transports the web to the dry end of the machine. In the dry end of a conventional machine, a press felt compacts the web into a single region, i.e., uniform density and basis weight, cellulosic fibrous structure prior to final drying. The final drying is usually accomplished by a heated drum, such as a Yankee drying drum.
One of the significant aforementioned improvements to the manufacturing process, which yields a significant improvement in the resulting consumer products, is the use of through-air-drying to replace conventional press felt dewatering. In through-air-drying, like press felt drying, the web begins on a forming wire which receives an aqueous slurry of less than one percent consistency (the weight percentage of fibers in the aqueous slurry) from a headbox. Initial dewatering takes place on the forming wire. From the forming wire, the web is transferred to an air pervious through-air-drying belt. This xe2x80x9cwet transferxe2x80x9d occurs at a pickup shoe (PUS), at which point the web may be first molded to the topography of the through air drying belt.
Additional improvements to the web manufacturing process include micropore drying, in which drying is driven primarily by capillary attraction and uniform distribution of air flow. Micropore drying, also known as limiting-orifice through-air drying, is particularly useful for removing interstitial water from the web. Micropore drying typically includes two drying phases. In the first phase, capillary attraction between water and fibers in the web is overcome by vacuum-induced capillary suction which draws the water into the fine capillary network of the micropore drying surface. In the second phase, the fine capillary network of the micropore drying surface helps to uniformly distribute the air that is passed through the paper web. By way of example, micropore drying is described in commonly assigned U.S. Pat. No. 5,274,930, issued Jan. 4, 1994 to Ensign et al.; and U.S. Pat. No. 5,625,961, issued May 6, 1997 to Ensign et al.; both patents hereby incorporated herein by reference.
Drying efficiency is an issue in all predrying processes. For example, in the process described in the 5,625,961 patent, the hot air passes through the drying belt first, then through the sheet. Water carried by the drying belt is partially evaporated, thereby reducing sheet drying efficiency. Production rates are thus impacted by the water-carrying characteristics of the drying belt.
In general, through-air-drying preferably dries the web between wet transfer and xe2x80x9cdry transfer.xe2x80x9d At dry transfer, the web is transferred to a heated drum, such as a Yankee drying drum for final drying. During this transfer, portions of the web are densified during imprinting to yield a multi-region structure. Many such multi-region structures have been widely accepted as preferred consumer products.
Over time, further improvements became necessary. A significant improvement in through-air-drying belts is the use of a resinous framework on a reinforcing structure. The resinous framework generally has a first surface and a second surface, and deflection conduits extending between these surfaces. The deflection conduits provide areas into which the fibers of the web can be deflected and rearranged. This arrangement allows drying belts to impart continuous patterns, or, patterns in any desired form, rather than only the discrete patterns achievable by the woven belts of the prior art. Examples of such belts and the cellulosic fibrous structures made thereby can be found in U.S. Pat. Nos. 4,514,345, issued Apr. 30, 1985 to Johnson et al.; 4,528,239, issued Jul. 9, 1985 to Trokhan; 4,529,480, issued Jul. 16, 1985 to Trokhan; and 4,637,859, issued Jan. 20, 1987 to Trokhan. The foregoing four patents are incorporated herein by reference for the purpose of showing preferred constructions of patterned resinous framework and reinforcing type through-air-drying belts, and the products made thereon. Such belts have been used to produce extremely successful commercial products such as Bounty paper towels and Charmin Ultra toilet tissue, both produced and sold by the instant assignee.
As noted above, patterned resinous through-air-drying belts use a reinforcing structure, the reinforcing structure preferably being an interwoven fabric. The reinforcing structure preferably provides sufficient rigidity to the belt, making it durable for papermaking. Without sufficient rigidity, the life of the papermaking belt is compromised, making frequent belt changes necessary. The cost of replacement belts, as well as the cost of the accompanying down time to the papermaking machine is unacceptable for commercial papermaking operations.
The reinforcing structure also has an important function of supporting the fibers fully deflected into the above-mentioned deflection conduits of the resinous framework, thereby enhancing web characteristics, for example, by minimizing pinholing in the web. Fiber support is characterized by a Fiber Support Index, or FSI, and reinforcing structures having an FSI as low as 40 have been found useful. However, to minimize pinholing and to provide a more uniform web surface, it is preferable to have an FSI of at least about 68. As used herein, the Fiber Support Index, is defined in Robert L. Beran, xe2x80x9cThe Evaluation and Selection of Forming Fabrics,xe2x80x9d Tappi Apr. 1979, Vol. 62, No. 4, which is hereby incorporated herein by reference.
Additionally, the reinforcing structure ideally has low void volume, thereby being low water carrying. By using a low water carrying reinforcing structure, more of the drying energy can be expended drying the paper web, and less expended drying the through-air-drying belt. While void volume and water carrying capacity do not perfectly correlate, in general, water carrying capacity is inherently limited by the available void volume. Therefore, by minimizing the void volume of the reinforcing structure, the water carrying capacity is necessarily minimized as well.
Early through-air-drying belts used a single-layer, fine mesh reinforcing element, typically having approximately fifty machine direction and fifty cross-machine direction yarns per inch. While such a fine mesh was acceptable from the standpoint of being low water carrying, and controlling fiber deflection into the belt (i.e., acceptable Fiber Support Index, as described below), it was unable to withstand the environment of a typical papermaking machine. For example, such a belt was so flexible that destructive folds and creases often occurred. The fine yarns did not provide adequate seam strength and would often burn at the high temperatures encountered in papermaking.
A new generation of patterned resinous framework and reinforcing structure through-air-drying belts addressed some of these issues. This generation utilized a dual layer reinforcing structure having two layers of machine direction yarns. A single cross-machine direction yarn system ties the two layers of machine direction yarns together. The dual layer reinforcing structure added rigidity and resulted in a much more durable belt, able to withstand the aforementioned environment of a typical papermaking machine. However, due to the nature of the weave, the belt caliper and void volume increased, causing the belt to carry much more water through the drying process, resulting in some drying inefficiencies during papermaking. Also, due to the weave pattern on the top layer, dual layer reinforcing structures did not always provide adequate fiber support (i.e., unacceptable Fiber Support Index, as described below), resulting in additional development to minimize undesirable paper characteristics, including pinholes.
Triple layer reinforcing structures were developed, the triple layer belts being essentially a two layer structure with each layer comprising machine direction yarns and cross-machine direction yarns (i.e., warps and shutes). In preferred embodiments, the top layer (i.e., web facing layer) is a square weave. The use of the square weave web-facing layer provides improved fiber support, and increased belt rigidity, as compared to dual layer belts. However, the void volume is higher than dual layer belts, resulting in high water carrying through-air-drying belts. Again, the high water content during processing results in additional energy costs to dry the paper web. Preferred triple layer belts are disclosed in U.S. Pat. Nos. 5,496,624, issued to Stelljes et al. on Mar. 5, 1996; and 5,500,277 issued to Trokhan et al. on Mar. 19, 1996; both patents hereby incorporated herein by reference.
Therefore, multiple layer structures offer sufficient belt rigidity, and may offer sufficient fiber support, but they generally contain high void volumes within the belt, which result in high water carrying capacity. This water content adds to the overall drying requirements of the papermaking process. Belt-carried water decreases the efficiency of through-air-drying processes, especially micropore drying where heated air typically encounters the belt-carried water prior to drying the paper webs. A significant amount of energy is expended to remove water trapped in the interstitial void volume of the belt prior to or during drying of the paper web.
The problem of belt-carried water, and the resulting drying inefficiencies, can be minimized by adding more yarns per inch woven in the same pattern, using monolayer reinforcing structures, using smaller diameter monofilaments in the weave, or combinations of the above. For example, fine-mesh, monolayer structures can be low water carrying due to their low thickness and minimal void volume. However, as mentioned above, such structures are not robust enough for commercial paper making. They are generally unable to withstand the environment of a typical papermaking machine, due to their relatively poor rigidity. Without a certain minimal amount of rigidity, the belt tends to wrinkle, or buckle, such that destructive folds and creases often occur at numerous points in its continuous path during papermaking. The constant bending, kinking, and local flexing quickly causes premature failure of the belt.
Dual-layer structures provide sufficient rigidity, resulting in increased belt life, and indeed are currently used for commercial paper production. However, as previously mentioned, dual layer belts tend to have relatively large void volumes within the reinforcing structure, thereby carrying excess amounts of water through the drying process. The excess amount of water can contribute to the overall energy costs associated with drying by limiting drying rates. Triple layer, and other multiple layer configurations also exhibit high water carrying reinforcing structures.
Accordingly, the prior art required a trade-off between low void volume (for low water carrying capacity) and flexural rigidity (for long belt life). In addition, the prior art required a tradeoff between high open area (for better through-air drying) and a fine mesh top surface weave of the reinforcing structure, (forming a monoplanar web facing surface for better fiber support).
The aforementioned approaches have not been entirely successful at achieving a desirable balance between belt void volume, fiber support, and belt rigidity. Clearly, yet another approach is necessary. The necessary approach recognizes that the web facing yams should provide maximum fiber support while the machine facing yarns should be configured to provide adequate rigidity for belt life, while only minimally impacting overall void volume.
Accordingly, it would be desirable to provide a papermaking belt that can reduce energy consumption in a paper making process.
Additionally, it would be desirable to provide a patterned resinous through-air-drying papermaking belt that overcomes the prior art trade-off of belt life and reduced water carrying capacity.
Additionally, it would be desirable to provide an improved patterned resinous through-air-drying belt having sufficient fiber support to minimize pinholing of a paper web, low water carrying capability, and sufficient durability to withstand the rigors of commercial papermaking.
Further, it would be desirable to provide an energy-efficient patterned resinous through-air-drying belt which produces an aesthetically acceptable consumer product comprising a cellulosic fibrous structure.
The present invention is a papermaking belt comprising two primary elements: a reinforcing structure and pattern layer. The reinforcing structure comprises a web facing first surface of interwoven first machine direction yarns and cross-machine direction yarns, the first surface having an FSI of at least about 68. The reinforcing structure has a machine facing second surface which comprises second machine direction yarns binding only with the cross-machine direction yarns in a N-shed pattern, where N is greater than four, wherein the second machine direction yarns bind only one of the cross-machine direction yarns per repeat. The pattern layer extends outwardly from the first surface, wherein the pattern layer provides a web contacting surface facing outwardly from the first surface, the pattern layer extending at least partially to the second surface.